1. Field of the Invention
The present invention relates to a television tuner circuit having VHF and UHF tuner sections.
2. Description of the Prior Art
A certain tuner circuit for television uses a band switching diode for switching the low and high bands of the VHF tuner section.
For selecting the VHF low band, the band switching diode is turned off so that the input tuning circuit enters a VHF low band receiving mode. For selection of the VHF high band, on the other hand, the band switching diode is turned on so that the input tuning circuit is placed in the VHF high band receiving mode. Further, for selection of UHF, the band switching diode is subjected to zero bias mode.
This tuner circuit has the advantage that the band-switching diode is operated, i.e., the band is switched with a single-polarity power supply.
If the band switching diode is placed in the zero bias mode for selection of the UHF band in the VHF or UHF tuner circuit, however, the VHF component of a large signal higher than 1 V reaching the VHF antenna input tuner circuit is applied to the band switching diode, with the result that the band switching diode generates a high harmonic or snow noise attributable to the non-linear rise characteristics of the diode. This component is radiated on the UHF antenna input tuner circuit of the UHF tuner section, thereby deteriorating the S/N ratio of the UHF signal, resulting in the lower quality of the picture.
In order to solve this problem, the conventional tuner circuits use a power supply of two polarities, positive and negative, for controlling the band-switching diode. During the receipt of the VHF low band, the band-switching diode is turned off by the positive power supply; during the receipt of the VHF high band, the band-switching diode is turned on with the positive power supply; and during the receipt of the UHF band, the band-switching diode is impressed with a reverse bias from the negative power supply, thus preventing the diode from generating any high harmonic or snow noises.
The conventional methods are unsatisfactory both technically and in cost since the band switching operation requires a power supply of two polarities, positive and negative, and the number of required power terminals of the tuner is increased, thus complicating the circuit configuration.